Wash the potatoes and, without peeling them, place them in a large pot. Cover with water and boil until you can easily insert a knife into them. Remove from the heat and let them cool.

Peel the potatoes and cut them into large cubes.

Slice the red onion into thin strips. Peel the apple and cut it into small cubes. Add both to the potatoes.

Combine the mayonnaise, mustard, olive oil, and balsamic vinegar. Mix well and add the dressing to the potatoes.